High-frequency signal correlator

ABSTRACT

An automatic frequency correlator computes auto and cross correlation functions of up to 3 GHz. bandwidth over 100 sampling points. The frequency response of the correlator of the present invention is an improvement of three orders of magnitude over that obtainable by prior art correlators, and this is accomplished by providing a new and more convenient means of generating the necessary delay times in the operation of the present correlator.

621 22 ll 16 71 r 3,621,223

[72] Inventors Igor Alexelf 3,428,794 2/1969 Norsworthy 235/181Oakridge; 3,5 14,585 5/1970 Norsworthy 235/181 Rodger v. Neidlgh,Knoxville, TCIIIL; FORElGN PATENTS William Ray Wlng, Pella, Iowa PP No.861,381 1,493,450 7/1967 France 235/181 22 Filed Sept. 26, 1969 OTHERREFERENCES [45] Patented Nov. 16,1971 Princeton Applied Research: SignalCorrelator Model 100 [73] Assignee The United States of America as W 3pages.

represented by the Unit d Stat At i Zimmerman: How to ExtendSampling-Oscilloscope Versa- Energy Commission bility IEEE SpectrumApril 1969 p. 79/85 Primary Examiner-Malcolm A. Morrison 54HIGH-FREQUENCY SIGNAL CORRELATOR jff fi gzi g Grub" lchlmsnnwing Flgsorney- 0 an n erson [52] U.S.Cl 235/181, 235/183, 235/150.53, 324/771-1, 328/151 [51] lnt.Cl G063 7/19 ABSTRACT: An automatic frequencycorrelator computes [50] Field of Search 235/181, auto and crosscon-elaticn functions of up to 3 I52; 128/2 width over 100 samplingpoints. The frequency response of the correlator of the presentinvention is an improvement of [561 References Cied three orders ofmagnitude over that obtainable by prior art UNITED STATES PATENTScorrelators, and this is accomplished by providing a new and 3,331,9557/1967 Norsworthy 235/181 more convenient means of generating thenecessary delay 3,333,091 7/1967 Masak 235/181 times In the operationofthe present correlator.

DISPLAY SWEEP OUT PULSE OSCILLOSCOPE 3E GENERATOR SAMPLE Q 1 SAMPLINGANALOGUE OUT OSCILLOSCOPE HULTIPLIER EXT TRIGGER 15 SAMPLE AND SIGNALHOLD SAMPLING AVERAGER OUT OSCILLOSCOPE EXT 39 I l TRIGGER IN 0 SIGNAL-GSIGNAL-F PATENTEDuuv 1s IBII 3,621,223

sum 1 or 2 I I F- l I I ARB. I UNITS V l I I :SignaIG. I I I L I I ARB.l UNITS I I I I Fig. .L

DISPLAY I PULSE OSCILLOSCOPE 27-- GENERATOR 11 I TRIGGER CIRCUIT 25TRIGGER I29 CIRCUIT TIME GEREIIIIIOR 13 I9\ SAMPLE ANALOGUE ANDMULTIPLIER HOLD SAMPLE CIRCUIT AND I HOLD I clmcun ,III I5 I WAVE FORMSIGNAL-G SIGNAL-F AVERAGER J mvsurons.

I gor Alexeff qdger V. Ne rd:gh BY WIIIIam. R. Wmg

ATTORNEY.

PATENTEmmv 1s |97| SHEETZUEZ SWEEPOUT DISPLAY EXT PULSE OSCILLOSCOPESWEEP GENERATOR 13 SAMPLE l QR SAMPLING ANALOGUE OUT OSCILLOSCOPEMULTIPLIER EXT TRIGGER 15 SAMPLE AND SIGNAL HOLD SAMPLING AVERAG ER OUT9 OSCILLOSCOPE EXT 1 (b mlccsn o SIGNAL-G SIGNAL-F Fig.3

AUTO-CORRELATED B-GIGAHERTZ SINE WAVE Fig.4

TURBULENT PLASMAJOO keV ELECTRONS,-800MEGAHERTZ CORRELATED FOR ONLY AFEW CYCLES- INVENTORS. Igor Alexeff Rqdger Nqldlgh BY William R. Wing 72.2 W1 (A. ATTORNEY HIGH-FREQUENCY SIGNAL CORRELATOR BACKGROUND OFINVENTION This invention was made in the course of, or under, a contractwith the US. Atomic Energy Commission.

The concept of computing correlation functions is not new.

1 For example, the US. Pat. to Van Horne, No. 2,840,308, is-

sued June 24, 1958, describes a typical prior art correlator. However,such prior devices are generally limited to relatively low frequencies;for example, 250 kHz. or less.

Studies over narrow bandwidths in the MHz and GH: range have grownincreasingly important in turbulent plasma research. The existence ofwaves that repeat over only a few cycles are now known to existin aplasma. For purposes of beam control and for identification of beamconditions, true correlators having the ability to correlatefluctuations beyond the commonly studied electron cyclotron and plasmafrequencies were, prior to the present invention, nonexistent. Thus,there exists a need for a correlator that is capable of not onlyfunctioning properly but also doing so accurately for frequencies in thegigal-Iertz range. The present invention was conceived to meet this needin a manner to be described below.

SUMMARY OF THE INVENTION It is the object of the present invention toprovide an improved correlator for automatically computing auto andcross correlation functions of up to a 3 GHz bandwidth.

The above object has been accomplished in the present invention byproviding two sample-and-hold circuits, a timedelay generator, a masterpulse generator which simultane' ously triggers the time-delay generatorand one of the sampleand-hold circuits, and, at the end of thecontrollable time.- delay cycle, the secondsample-and-hold circuit istriggered. The outputs of the two holding circuits are then applied toan analogue multiplier until it has has sufficient time to multiply themaccurately, and then the multiplier output is connected to a displayoscilloscope by way of an integrator or signal averager. The next pulsethen arrives from the master generator and the process repeats. Thissystem is adapted to correlate signals in' the gigal-Iertz range, in amanner to be described hereinbelow, which was not possible to achievewith prior art correlators. In the past, correlators have been capableof broad frequency ranges-but low maximum frequency capability.Automatic-scanning delay lines to provide delays to high frequencies maybe built, but characteristically they operate only over a narrowfrequency range. The present system eliminates the automatic scanningdelay line. In the present system, the two sample-and-hold circuits andthe time-delay generator provide for an equivalent operation while atthe same time providing a correlator which has an improvement infrequency response of three orders of magnitude over any prior artcorrelator.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph of typicaloscilloscope traces from av DESCRIPTION OF THE PREFERRED EMBODIMENTS Thecorrelation method, uponwhich the present invention is based, isdescribed by the following known equation:

This summation formula is explained with the aid of FIG. 1. In FIG. 1there are depicted the representations of two signals, F and G, whose'cross correlation function, I (r), is being computed. The figureillustrates sample pairs (one from each signal, each having a width A!)being taken; a precise time interval 1' separates the elements of. eachpair. Other sample pairs are taken at later time intervals. Each samplepair is multiplied and all pairs are averaged to produce thevalue of thecorrelation function corresponding to the delay time (7) between samplepairs.

.For the above method to have meaning, it is essential that the signalsF and G are stationary in time. For example, if F and G representvariables-in a turbulent plasma, the turbulence must be homogeneous on atime scale comparable to or longer than the integration time (timerequired for n samplings) of the correlator. The order of elements inthe sum of the above formula is immaterial and, if the turbulence istruly homogeneous, the elements I, are indistinguishable. Thus, thesampling process can proceed at random with respect to the phase of anysignals of interest. In this way, the correlation function can be builtup bytaking timed pairs of samples. The sample length is At, the timebetween pairs is n, t,), and the time 7 separates the elements ofa-pair. Many such pairs must be taken, multiplied, and averaged toproduce the correlation function for each value of r. Of course, if thefunction being generated is autocorrelation, the sample pairswouldbe-takenfrom one signal instead of from two signals as in FlG.l.

The above method of correlation can be accomplished with the deviceshown in FIG. 2. For any particular taking of a sample pair, the actionis started by a pulse from a pulse generator 27 which simultaneouslyactivates two trigger circuits 2! and 29. Immediately, a sample is takenfrom signal F by sampleand-hold circuit 3ljand held on .one, input toanalogue multiplier. 13. The sampling of signal G is delayed therequired time, r, by the time-delay generator 25. When the time, 1, haselapsed, sample-and-hold circuit l9ssamples signal G and applied it tothe second input of analogue multiplier 13.

Analogue multiplier 13 executes the multiplication 'of the samples andthe resulting value is applied to the waveform averager 15; If pulsegenerator 27 is operating at a repetition rate of 1 kHz., for example,there isample time for the abovementioned sample pair to be taken,multiplied, and averaged. At the initiation of the next and succeedingpulses from pulse generator 27, theprocess repeats and theresultingaveraged value of the correlation function is generated in waveformaverager 15. In this configuratiomthe correlation function is generatedpoint-by-point on the display oscilloscope II. The horizontal sweep ofoscilloscope 11 is slow compared to all other time constants of thesystem. The vertical deflection of the display oscilloscope beam isproportional to the averaged output of the multiplier 13. The time-delaygenerator 25 is then stepped up to a new value, of r for-the computationof the correlation function of that 'r. The horizontal sweep output ofthe display oscilloscope ll-is used for this stepping function. Thesawtooth voltage is so slow compared with other time constants of thesystem that it'may be regarded as a long series of infinitesimalsteps,,each step being a point on the final computed correlationfunction; the steps being infinitesimal, the points blend and thecorrelation function is continuous.

The system of FIG; 3 illustrates another embodiment of the presentinvention which is different from the embodiment of FIG. 2 in thefollowing respect. It is different in only one sense. The relative orderof time scale for display and averaging may be interchanged. This ispermitted if the signal averager I5 is capable of performing manyChannels of independent averaging simultaneously. For example, a PARWaveform Eductor, which may be used as the signal averager l5, handleschannels independently, and a Northern Scientific digital oscilloscopewill average 500 channels independently. In addition to the signalaverager IS, the system of FlG. 3 include two sampling oscilloscopes 37and 39, a pulse generator 27, and analogue multiplier l3, and a displayoscilloscope ll.

The sampling oscilloscopes are not used in their normal configuration;only the time-delay generator and sample-andhold circuit of oscilloscope37 are used, and in oscilloscope 39 only the sample-and-hold circuit isused (the delay is bypassed by setting the horizontal sweep on Manual").Pulse generator 27 simultaneously triggers the time-delay generator insampling oscilloscope 37 and the sample-and-hold circuit in samplingoscilloscope 39. At the end of the time-delay cycle, the sample-and-holdcircuit in oscilloscope 37 is triggered, causing signal G to be sampled.The sample-and-hold outputs of the two sampling oscilloscopes areapplied to analogue multiplier 13 until it has had sufficient time tomultiply them accurately. The next pulse then arrives from the mastergenerator and the process repeats. It should be noted that the sweepoutput, the sawtooth of the display oscilloscope 11, is connected to thesignal averager 15 to allow it to sweep through its channels ofaveraging in step with the display oscilloscope 11. In other words, thesweep output from the display oscilloscope 11 function as asynchronizing pulse input to the signal averager 15.

In order to obtain an increasing delay in oscilloscope 37 in a propersequence, the sweep circuit is set on External" and it is driven by thesweep of display oscilloscope 11. In this case the 1- increments becomeinfinitesimal and the resolution is limited only by the method ofaveraging chosen 100 channels in the case of the Waveform Eductor). Asmentioned hereinabove, the synchronizing pulse from the sweep outputfrom the display oscilloscope to the signal averager 15 allows the unitT to sweep through its channels of averaging in step with the displayoscilloscope 11. This sweep, being coordinated with the displayoscilloscope, provides that the desired range of T is scanned for eachsweep of the display oscilloscope, with 1 being constant for a shortincrement of time. lf, in oscilloscope 37, the equivalent sweep speedhas been set for one nanosecond per cm., for example, and the beam is 5cm. from the screen edge, then a 5-nsec. delay is generated between thepulse generator signal arrival and the sample being taken. ln this casethe many sample pairs will be multiplied by the unit 13 and averaged bythe unit 15 for this value ofr to be applied to the vertical deflectionof oscilloscope 11 as one dot. The value of-r is then increased and inthis way a chain of values of the correlation function, each for asucceedingly larger time-delay 1', will be generated, displayed, andheld on the face of display oscilloscope 11.

Typically, in this oscilloscope embodiment, the delay can be ranged from100 usec. to 1.0 nsec. It is also typical in oscilloscopes to be able toselect, for example, 100 values of the vertical deflection to bedisplayed in a single sweep of the beam.

The number of samples and the width of the oscilloscope trace set anupper and lower bound on the bandwidth of the correlation function thatcan be examined at any one setting. If the lowest usable frequency isarbitrarily defined as one cycle per trace length, then, at anyparticular equivalent sweep speed, the greatest usable frequency will beabout 20 times the lowest. With the range ofsweep speeds provided, the

lowest frequency can vary from kHz. to 500 MHz. However, at the highestequivalent sweep speeds, the greatest usable frequency is limited by theresponse time of the circuits, not the number of channels. The 3 db.point is reached at 875 MHz and the frequency response rolls offsmoothly above this, although it is still usable well beyond 2,000 MHz.

In the system of FIG. 3, each ofthe sampling units 37 and 39 may be, forexample, a Tektronics Model 564 oscilloscope equipped with one Model 381Dual-Trace Sampling Unit and a Model 3T77A Sampling Sweep Unit. Thepulse generator 27 may be, for example, a General Radio Model l,2l7BPulser for generating the clock drive pulse, and the signal averager maybe, for example, a Princeton Applied Research Waveform Eductor." Themultiplier 13 may be, for example, an Optical Electronics, lnc., Model5,109 unit. and the final display unit 11 may be, for example, aHewlett-Packard Model l75A oscilloscope. lt should be understood thatequivalent units of other manufacturers could be used, if desired, in asimilar manner.

The circuit shown in the block diagram in H0. 3 is by no means the onlyone possible. The sweep output of the sampling oscilloscope 37 couldjust as well be used to drive the display scope ll, or the ramp from thesignal averager 15 could drive them both. Depending on the gainavailable in the multiplier and averager, it might be necessary toinsert amplifiers after these stages. The signal averager could bereplaced by either a passive RC network or an operational amplifier usedas an RC integrator. Such an RC integrator could increase the resolutionan order of magnitude beyond the channels offered by the signalaverager. On the other hand, any such RC network must have a timeconstant that is a small 2 percent) fraction ofthe sweep time to avoidsmearing any high fourier components.

It should be understood that the extremely slow clock rate, 1 kHz., forexample, is chosen only partly because of frequency limitations of themultiplier. The output square waves from the sample-and-hold circuitsarrive at the multiplier out of phase with each other by the value of -rat any particular instant. During this time 1 the output of themultiplier is a signal of random amplitude which will average to zero inthe integration process. As a result, the amplitude of the correlationfunction being generated in the averager will be decreased by apercentage equal to the fraction of the holding timer represented by -r.The effect, of course, would be a linear decrease in amplitude from theleft to the right edge of the trace. It could be compensated for, but itis simpler to merely hold the maximum value of'r to less than I or 2percent of the clock time.

Operation of the correlator has been described as though all triggeringand sampling operations took place in real time. They do not, and anunderstanding of this is important. The trigger circuits of the sweepunits of the sampling oscilloscopes achieve their precise triggeringability at the expense of considerable delay. To compensate for this,the vertical amplifiers contain fixed delay cables which provide a delaygreater than that introduced by the trigger circuits. The triggercircuits themselves then, contain a variable standoff time that isadjusted from a front panel control to place a pulse in the center ofthe screen at whatever sweep speed is being used. The variablestandoff-time controls permit the trigger circuits of the two samplingoscilloscopes to be adjusted so that they are exactly synchronized.Variations in the delay of the two trigger circuits or variations in theeffective lengths of cable used to deliver trigger pulses can thus becompensated for in the system. This process is accomplished by simplyapplying the same fast 1 nsec.) rise-time pulse to both oscilloscopesand centering it independently on each screen, This adjustable standofftime also permits placing the time 1-0 at any point on the screen (oroff of it), allowing the correlation function to be examined bothforward and backward in time. Since by definition l (r) must besymmetric about -r=0, the symmetry (or lack of it) can be used as aready check on both the operation of the system and for the presence ofwhite noise in the correlation function.

FIGS. 4 and 5 are pictures taken from photographs that show what thesystem is capable ofdoing. H0. 4 is an autocorrelated 3 GHz sine wavesignal at an equivalent sweep speed of 0.1 nsec./cm. it was impossibleto photograph the original 3 GHz output of the oscillator because thefrequency is well above the point at which a sampling oscilloscope willdeliver a stable trace. The relatively slow time period over which thecorrelator operates, however, was sufficient to provide the display ofthe correlation functions.

FIG. 5, also autocorrelation, shows the presence of an-800 MHzoscillation in a turbulent plasma. From the picture it can be seen thatthe signal possesses correlation over a few cycles.

Although the present invention was primarily developed for use in plasmaphysics. It can perform correlations expected to be of value inmicrowave and radar signal transmission studies and interplanetary noisesignal studies. In plasma physics it would be possible for the presentinvention to function as a control monitor. For example, it is wellknown that, in plasma situations, oscillations are always present andthe present invention would be useful in examining plasma for the natureof its oscillations.

The high-frequency response and the flexibility of the invention allowstudies to be made quickly in various applications and over a frequencyrange extended beyond that available in prior art correlators. It shouldbe apparent also that the various components of the system asillustrated in FIG. 3 are not altered internally and can beconventionally used when desired for other applications.

It should be noted that the 3 Gl-lz limit of the present invention isset by the sampling speed of the particular commercial units utilizedtherein as identified hereinabove, When faster sampling oscillatorsbecome available, correlations at higher bandwidths will be possiblewith the system of the present invention.

The present invention has been described by way of illustration ratherthan by way of limitation and it should be apparent ta it is equallyapplicable in fields other than those described.

We claim:

I. An automatic high-frequency signal correlator comprising a pulsegenerator, a first sampling oscilloscope provided with a firstsample-and-hold circuit, said first oscilloscope having its horizontalsweep thereof set on Manual, a second sampling oscilloscope providedwith a time-delay generator and a second sample-and-hold circuit, saidfirst and second sampling ocilloscopes coupled to said pulse generatorfor activation of said first sample-and-hold circuit of said firstsampling oscilloscope and activation of said time-delay generator ofsaid second sampling oscilloscope, the output of said timedelaygenerator being coupled to said second sample-and-hold circuit, a firstsource of signals to be sampled and being coupled to said firstsample-and-hold circuit, and analogue multiplier, the output of saidfirst sample-and-hold circuit coupled a one input to said analoguemultiplier, a second source of signals to be sampled and coupled to saidsecond sample-andhold circuit, the output of said second sample-and-holdcircuit coupled as a second input to said analogue multiplier, a signalaverager coupled to the output of said analogue multiplier, a displayoscilloscope coupled to the output of said signal averager, the sweepoutput of said display oscilloscope and as sweep input to said signalaverager such that the sweeps are coordinated and the delay time fortaking of samples will increase periodically in increments, whereby aplurality of output signals from said signal averager are displayed bysaid display oscilloscope to provide a visual correlation of the samplesignals from said first and second source of signals.

l K l i 2% UNITED STATES PATEN'I OFFICE CERTIFICATE OF CORRECTION PatentNo. 3,62l,223 Dated November is; 1971 Inventoi:(s) Igor Alexeff et a1 Itis certified that error appears in the above-identified patent: and thatsaid Letters Patent are hereby corrected as shown below:

Column l line 3, before "INVENTION", insert --THE---; line 36, delete"has", one occurrence; in the equation at line 74, over "2" "11" shouldread ---n---.

Column 2, line 23, "(t should read ---(tlines 38 and 39, "applied"should read ---applies---; line 5 before "proportional" insert ---(ateach point)---; line 55, after "output" insert ---(the sawtooth)---;line 66 "scale" should read ---scales--,-; line 73, "include" shouldread ---includes---.

Column 3, line l8, after "sweep" insert ---(gate)---; line 19 "function"should read ---functions---.

Column 4, line 53, "T 0" should read ---1 0---.

Column 5, line 2l "ta" should read ---that---.

Column 6, line 4, "ocilloscopes" should read ---oscilloscopes---; linel0, "and" should read -'---an---; l ine 12, "a" first occurrence, shouldread ---as---; line 18, after "oscilloscope" insert ---coupled as anexternal sweep input to said second sampling oscilloscope---; line 20,after "for", insert ---the---.

Signed and sealed this 1 th day of July 1972.

(SEAL) Attest:

EDWARD M.FLETCHER, JR. ROBERT GOTTSCHALK Attesting OfficerCommissionerof Patents

1. An automatic high-frequency signal correlator comprising a pulsegenerator, a first sampling oscilloscope provided with a firstsample-and-hold circuit, said first oscilloscope having its horizontalsweep thereof set on ''''Manual, '''' a second sampling oscilloScopeprovided with a time-delay generator and a second sample-and-holdcircuit, said first and second sampling ocilloscopes coupled to saidpulse generator for activation of said first sample-and-hold circuit ofsaid first sampling oscilloscope and activation of said time-delaygenerator of said second sampling oscilloscope, the output of saidtime-delay generator being coupled to said second sample-and-holdcircuit, a first source of signals to be sampled and being coupled tosaid first sample-and-hold circuit, and analogue multiplier, the outputof said first sample-and-hold circuit coupled a one input to saidanalogue multiplier, a second source of signals to be sampled andcoupled to said second sample-and-hold circuit, the output of saidsecond sample-and-hold circuit coupled as a second input to saidanalogue multiplier, a signal averager coupled to the output of saidanalogue multiplier, a display oscilloscope coupled to the output ofsaid signal averager, the sweep output of said display oscilloscope andas sweep input to said signal averager such that the sweeps arecoordinated and the delay time for taking of samples will increaseperiodically in increments, whereby a plurality of output signals fromsaid signal averager are displayed by said display oscilloscope toprovide a visual correlation of the sample signals from said first andsecond source of signals.